Production television systems have become very prevalent in the art. While tube-type television systems can actually produce better resolution and more brightness, the size of such projection tubes has been limited by cost and weight. At the present stage of development of television receivers, a 35-inch projection television may cost one-tenth of the price of a 35-inch tube-type television system. Therefore, such projection receivers are very useful, and have gained wide user popularity.
A projection television receiver synthesizes images on a screen by projecting these images from three tube projectors. Each of the three tube projectors projects one monochromatic image corresponding to one of the three primary colors--red, green and blue. However, the convergence between the three tube projectors has a tendency to be influenced by terrestrial magnetism. Due to the large viewing surface and the necessity for accurate viewing synchronization, it is very important to converge these red, green and blue scans.
It is therefore required to correct the convergent deviation of each scanning line using a convergence adjustment. This deviation can be created by terrestrial magnetism, by changing the direction of the projection cathode ray tube, or by adjusting a posture thereof.
FIG. 1 illustrates a conventional digital convergence circuit 1 used in a projection type television receiver 4. This television projects images on a front-facing screen 3 from monochromatic projection RGB cathode-ray tubes 2r, 2g, and 2b. Horizontal and vertical convergence coils (not illustrated) are provided for each of the projection cathode-ray tubes 2r, 2g and 2b, and are incorporated in a convergence deviation correcting circuit 5. Storing circuit 6 is connected to convergence deviation correcting circuit 5. Storing circuit 6 stores correction data necessary for correcting the convergent deviation of each of the sampling points in the matrix on screen 3. This data has been previously correlated and is stored in storing circuit 6. The reading of the correction data from storing circuit 6 is controlled by a readout control circuit 7 to be synchronized with deflection scanning, so that the correction factors are supplied to the convergent deviation correcting circuit 5 synchronized with deflection scanning.
In order to adjust the convergence of this kind of prior art digital convergence circuit 1, adjustment images of, e.g., a crosshatch pattern or a dot pattern are projected on the screen 3. Then, correction data required for correcting the convergent deviation is collected and stored in storing circuit 6. The optimal correction data must, however, be obtained on a trial-and-error basis while varying the correction data supplied to the convergent deviation correcting circuit 5 for every sampling point. For this reason, a long operating time is necessary for this process. Moreover, an increase in the capacity of storing circuit 6 becomes inevitable as the sampling points increase in number. In some projectors--especially a multi-scan projector-type in which a deflection mode is automatically changed-over corresponding to horizontal deflection frequencies of 15 to 4l kHz-wave filtering characteristics of the convergence coils and their driving circuits, as well as characteristics of a deflection circuit vary according to the horizontal deflection frequencies. This correspondingly causes the correction data that needs to be stored in the storing circuit 6 to become different for the different horizontal frequencies. This requires a still larger capacity of storing circuit 6.
The conventional digital convergence circuit 1 uses readout control circuit 7 to generate address signals in accordance with clock signals obtained by multiplying or dividing frequencies of the horizontal synchronization signals, while synchronizing with these signals. If the correction data is necessary for the correction of convergence deviation, the correction data stored in the address designated by the address signals are read from storing circuit 6. The multi-scan projector therefore has numerous problems. For one, when a scanning line density changes from a high level density to a low level density, and the correction data are read in accordance with the address designated by readout control circuit 7 in a conventional mode, some correction data remains unread at the end of the picture scanning. The correction data originally has a vertical symmetry with respect to the center of the picture, and has a symmetric center with respect to portions other than the center of the picture. As a result, the convergence correction does not completely correct the convergence.
It is therefore a primary object of the invention to obviate the above-described problems by providing a digital convergence circuit capable of computing correction data within a short period of time which are required for correcting convergent deviation of a scanning line of scanning a picture.
To this end, according to one aspect of the invention there is provided a digital convergence circuit capable of correcting the convergent deviation of the scanning line in accordance with variations in number of scanning lines.
This digital convergence circuit includes a coefficient storing structure that stores weighting coefficients for each of a plurality of fundamental correction waves. These fundamental correction waves have either a horizontal scanning period or a vertical scanning period. The fundamental correction waves are waves that can be weighted by weighting coefficients stored in the coefficient storing means, in order to form synthesized waves. The synthesized waves are then representative of the correction data that is used to correct a deviation of a scanning line of the picture. The coefficients stored in the coefficient storing means are then used along with the fundamental correction waves to form correction data. This correction data is stored in a correction data storing device. The correction data is read from the correction data storing device in synchronism with the deflection scanning, and the convergence of the scanning line is corrected. The fundamental correction waves can include parabolic undulation waves with a vertical or horizontal scanning period, serrated undulation waves, parabolic waves amplitude modulated by the serrated waves or serrated waves amplitude modulated by the parabolic waves.
Another aspect of the invention includes a correction data storing device which stores correction data for correcting a convergence deviation of a scanning line. This correction data is stored at addresses corresponding to locations in the picture. An address generating device produces addresses to address this correction data in synchronism with clock pulses that are obtained from a synchronization signal of the picture. A readout starting address of the correction data storing device is determined in accordance with a number of scanning lines. This enables obtaining a same correction data at the center of the picture, independent of the number of scanning lines. As such, vertical symmetry of the correction data can be ensured.
According to yet another aspect of the invention, correction data storing means stores data for each of a plurality of sampling points that split the picture into a matrix. A readout address generating device designates an address of the correction data storing device from which to read. The central processor computes the correction data and writes the data to the correction data storing device at addresses corresponding to locations on the picture. The correction data is computed using a correction function which includes a coefficient that is variably set and a variable indicating deflection scanning position when adjusting the convergence. This aspect of the invention also splits one horizontal scanning period into a plurality of access periods. The plurality of access periods correspond to a number of sampling points arranged in a horizontal direction of the picture scanning. These access periods are divided into a first period which is a time required to address the address generating device and a second access period which is an access period of the central processor to the correction data storing device. According to yet another aspect of the invention, this latter addressing period is a fixed period which does not fluctuate with the horizontal scanning frequency.